The present invention relates to the injection of a precise quantity of gas into a system.
The processes used to place a predetermined mass of a gas into a system have remained virtually unchanged for decades. Generally, the mass of a gas within a system is determined to one of a gravimetric process, a partial pressure process, or an analytical process.
The gravimetric process involves, for example, weighing an empty cylinder with a known volume in which the gas or gas mixture is to be placed. Gas is pumped into the cylinder and, thereafter, is weighed a second time. The weight of the empty cylinder is subtracted from the weight of the partially full cylinder to determine the mass of the gas contained within the cylinder. This partial filling of the container and weighing of the container is repeated until the cylinder contains the desired mass of gas. The concentration of the gas is derived from the mass of gas within the container, volume of the container, and the density of the gas. The accuracy of the determination of the mass of gas contained within the cylinder is dependent upon the accuracy of the weighing apparatus used and the accuracy of each measurement of mass. Multiple measurements of the partially filled container must be made for even relatively simple and common concentrations of gas. Obviously, this is a crude and a relatively time consuming method by which to place a precise amount of gas within a closed system, such as a cylinder. The accuracy of the gravimetric process is generally limited to a maximum of five percent for a typical gas. However, when it is desired to fill a cylinder with a very low mass of gas, or a low concentration of one or more gases, this method provides an accuracy of only about ten percent.
The partial pressure method involves Daltons law, which states that the total pressure of a mixture of gases is equal to the sum of the pressures of all of the component gases taken separately. Daltons law, however, holds true only for ideal gases. Furthermore, where a low concentration of one or more component gases in a cylinder is desired, it is difficult to obtain a high degree of precision using the partial pressure method since the pressure of a low concentration of gas in a given volume is relatively small and can be obscured by a gas present in higher concentration which, therefore, exerts a greater pressure.
The analytical process involves analytically monitoring the proportion or concentration of a gas within a system. Although this method provides an accurate measure of the concentrations of gases within the volume of a system, the concentration of the gases is determined after a mixture has been produced. If one or more of the concentrations of the component gases are not within a predetermined tolerance range of the intended concentration, the entire mixture must either be scrapped, reprocessed, or sold as a higher tolerance, and less profitable, mixture. Furthermore, the analysis of the mixture is a time consuming and expensive process, during the completion of which the quality of the product or mixture is unknown. Until the analysis is completed, the mixture can not be sold but rather must be stored by the manufacturer.
Mechanical critical flow orifice (CFO) kits are used to measure a flow of gas. CFO kits operate on a sonic nozzle principle. Critical orifice flow is achieved when the velocity of the gas through a CFO reaches the speed of sound (i.e., becomes sonic), and remains constant. Variables in critical orifice flow measurement calculations are avoided only if the flow through the CFO remains in the critical flow or sonic range. The critical rate of flow for a CFO is proportional to the ratio of the absolute static pressure at the nozzle inlet and the ambient temperature. For a given nozzle, there is a minimum ratio of pressure to temperature below which the flow rate of the gas through the CFO is no longer accurately predicted by the ratio of pressure to temperature under which the CFO is being operated. Manipulation of the inlet pressure is far simpler and more cost effective than varying the ambient temperature under which the gas is flowing through a CFO. Thus, in order to maintain the ratio of pressure to temperature above the minimum level at which the flow rate of the gas through the CFO is predictable, the pressure at the inlet of the CFO must be above a certain level. Typically, CFOs require an inlet pressure of at least about 20 to 30 psi in order to ensure operation of the CFO is critical. Such a relatively high inlet pressure makes it difficult to deliver a very low mass of gas to an external system, or cylinder. Furthermore, precisely machined and manufactured CFOs operating under critical conditions achieve a maximum flow rate accuracy of only about 1%.
A practical application of the ability to place a very low concentration of gas into a system arises in the testing of automobile emissions control systems. Automobile emissions are said to be the single greatest source of pollution in numerous cities across the country. Automobiles emit hydrocarbons, nitrogen oxides, carbon monoxide and carbon dioxide as a result of the combustion process. Evaporative emissions occur through the evaporation of gasoline in the engine and fuel tank. The automobile emissions control systems of today are so advanced and efficient that evaporative emissions, rather than emissions from the combustion process, can account for a majority of the total hydrocarbon pollution on hot days.
The Clean Air Act of 1970 and the 1990 Clean Air Act set national goals of clean and healthy air for all and established responsibilities for industry to reduce emissions from vehicles and other pollution sources. The 1990 law further tightened the limits on automobile emissions and expanded Inspection and Maintenance (I/M) programs to allow for more stringent testing of emissions. Standards set by the 1990 law limited automobile emissions to 0.25 grams per mile (gpm) non-methane hydrocarbons and 0.4 gpm nitrogen oxides. The standards are predicted to be further reduced by half in the year 2004.
Manufacturers of automobiles and emissions systems have risen to the challenge of reducing automotive emissions by designing Low-Emission Vehicles (LEVs), Ultra-Low-Emission Vehicles (ULEV), Super-Ultra-Low-Emission Vehicles (SULEVs) and Zero-Emission Vehicles (ZEVs). In particular, LEVs reduce the emissions by up to seventy percent, ULEVs reduce emissions by up to eighty-five percent, and SULEVs reduce emissions by up to ninety-six percent. For example, the emission requirement for a ULEV is that it emit no more than 0.04 grams of hydrocarbon per mile. A SULEV must emit no more than 0.01 gpm of hydrocarbons. The emission levels of these vehicles have been reduced to a level which even the most sophisticated equipment in a laboratory environment can not accurately measure. Furthermore, the emission levels have been reduced to a level which would require the I/M programs to use similarly sophisticated equipment at numerous testing locations, thereby rendering the I/M programs impractical and cost prohibitive. Corroborative of this fact is that Americas car companies have signed agreements with three Department of Energy national laboratories to develop prototype instruments which are capable of providing reliable, accurate, and high-speed measurement of the trace emissions from such vehicles.
These prototype instruments will require testing and calibration, a process which is rendered susceptible to inconsistent results and inaccuracies due to the minute levels of pollutants the instruments must detect. Typically, testing of instruments used in measuring emissions are themselves tested and/or calibrated by creating a flow of a precision mixture of gases, thereby simulating the exhaust of an ULEV vehicle, or by filling a Sealed Housing for Evaporative Determination (SHED) with a precision mixture of gas. The instrument under test is used to measure the known and precise mixture of gas and the measured results are then compared with the known composition of the gas. An accuracy parameter for the instrument under test can then be determined.
A typical, ULEV currently in production emits no more than about 1 part per million (ppm) of hydrocarbon once the catalytic converter has reached its operating temperature. The above-described conventional methods of dispensing a given mass of gas are not capable of accurately and repeatably creating a gas having a concentration of 1 ppm of hydrocarbon, and therefore are not capable of simulating the exhaust gas concentration of an ULEV or SULEV. Furthermore, the conventional methods described above are not capable of delivering a mass of gas which is low enough to result in the gas having a very low concentration, which is hereby defined to be below about 20 ppm, in a reasonably small volume. More particularly, most emissions testing laboratories use a CFO to dispense propane at room temperature. As described above, the critical flow rate of a CFO is determined in part by the ratio of the absolute static pressure at the nozzle inlet to the ambient temperature. For a given nozzle, this ratio must be kept above a predetermined minimum to maintain the critical flow. Emissions testing is typically performed at room temperature. Therefore, the only remaining variable for a given nozzle is its inlet pressure. Because of the low concentrations of undesirable gases emitted from ULEVs and SULEVs, simulating the exhaust of such a vehicle or filling a SHED with a gas having such a low concentration requires a very low flow rate from the CFO. Thus, either a smaller nozzle must be used or the inlet pressure must be reduced. The use of a smaller diameter nozzle is limited by machining tolerances. The use of an inlet pressure that is low enough to achieve such a low concentration of a component gas results in the ratio of pressure to temperature falling below the minimum ratio at which the flow rate through the CFO is predicted by the sonic principle. Thus, a CFO based on the sonic principle is not capable of injecting into a system or sealed enclosure a mass of gas which is small enough such that the gas will have a very low concentration. Therefore, a CFO is of little, if any, practical use in creating a gas having a concentration low enough to be of practical application in the testing and/or calibration of equipment intended for the measurement of emissions from a ULEVs and SULEVs.
The code of federal regulations requires that emissions testing laboratories perform a quality check on the equipment used in testing emissions. This test allows for an error of plus or minus two-percent in the concentration of a gas injected into a constant volume system or SHED. When testing and/or calibrating for a gas concentration of, for example, 30 ppm in the SHED, a two-percent error constitutes an error of 0.6 ppm in the concentration of the gas injected into the constant volume system or SHED. This same 0.6 ppm error, when testing at the level of, for example, 1 ppm, constitutes sixty-percent of the 1 ppm test level.
Therefore, what is needed in the art is an apparatus and method which enable the precise injection of a very low concentration of at least one gas into a system.
Furthermore, what is needed in the art is an apparatus and method which enable the precise injection of at least one gas into a system at a constant, predictable, and very low rate of flow.
Moreover, what is needed in the art is an apparatus and method which enable the precise injection of at least one gas into a closed system in approximately the same concentration as the concentration of undesirable gases contained in the exhaust flow of an ULEV and SULEV, thereby allowing testing and/or calibration of instruments intended to measure such low concentrations of gases.
Even further, what is needed in the art is an apparatus and method which enable the creation of a precise flow of a particular gas having approximately the same concentration of that particular gas as does the exhaust of an ULEV and SULEV.
The present invention provides an ultra accurate gas injection system for injecting at a precise flow rate a flow of gas into an external system, thereby producing a precise concentration of the gas in the external system.
The invention comprises, in one form thereof, an input device for inputting setpoint data including a target flow rate for the flow of gas into the system. The gas flows through a mass flow controller at an actual flow rate. The mass flow controller issues a flow rate signal indicative of at least the actual flow rate and receives a flow control signal. The mass flow controller is configured to control the actual flow rate dependent at least in part upon the flow control signal. A programmable controller is electrically connected to the input device and the mass flow controller. The programmable controller receives the setpoint data from the input device and issues the flow control signal, the flow control signal being dependent at least in part upon the target flow rate contained within the setpoint data. The programmable controller repeatedly reads the flow rate signal and compares the actual flow rate with the target flow rate. The programmable controller adjusts the flow rate signal dependent at least in part upon the comparison of the actual flow rate with the target flow rate, and is configured to adjust the flow control signal such that the actual flow rate is substantially equal to the target flow rate. The programmable controller issues an output signal dependent at least in part upon the flow rate signal. An output device is electrically connected to the programmable controller and receives the output signal. The output device indicates the actual flow rate.
An advantage of the present invention is that precise quantities of a gas are injected into an external system at a controlled flow rate. Thus, a very low concentration of gas can be injected into the external system with great accuracy.
Another advantage of the present invention is a user can inject at least one gas at a very low and accurate rate of flow, thereby achieving a very low concentration of gas in the external system.
Yet another advantage of the present invention is the flow rate of the gas into the external system can be constant or can vary in a predetermined manner as set by the user.
A further advantage of the present invention is a user can enter a time period for the injection of the gas into the external system, and the injection of gas will automatically cease upon the expiration of that time period.
A still further advantage of the present invention is the user can select and adjust the rate of flow of the gas into the external system.
An even further advantage of the present invention is that it allows testing and calibration of an instrument capable of measuring very low concentrations of gas.
Lastly, an advantage of the present invention is that it allows the injection of multiple gases at a fixed or user defined variable flow rate into an external system.